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Thank you for the graphs. Unfortunately, it is not that easy to find the root cause. We plan to make an extended debugging in BTSync targeted to debug speed issues, including saw-shaped traffic. I'll update the thread once the version is ready.

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I'm seeing similar issues when syncing large files. Speed goes up to reasonable level, then slows back down to zero, then picks back up again. Sometimes it pauses for some seconds while with "Nothing being transferred right now" in the Transfers tab, possibly before moving onto the next chunk. It's very inefficient as a result.

The box sending the files is a Debian 7 64-bit host, reciever is Win 8.

Also can you clarify whether Bt Sync uses multiple concurrent connections to transfer single large files? If not, when is this capability expected to be added? It would be a very useful feature for this software to have.

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Currently we utilize one logical connection, but it can be used to transfer several files simultaneously (if files are small). In your case - are your Debian and Win8 located in one LAN or they are connected over WAN?

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And the reason that I ask about multiple connections for large files is to do with the speed of international transfers. For whatever reason, one connection, or to put it more generally, one transfer over tcp cannon hope to saturate the width of the pipe between the two parties. So to work around this 'segment' download managers were created to be able to run multiple concurrent transfers for the one file, which allowed for a much greater utilisation of the bandwidth between the two parties.

It doesn't sound like BT sync is currently implementing such a system for single large files, so I ask again whether it is on the roadmap?

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Currently BTSync supports only one connection (if it connects over TCP), while the term connection is not applicable if we are talking about UDP.

The issue with speed is known and we are going to prepare a special debug build, which will allow us to find out the root cause of a low speed in some cases. I'll update this topic once we are ready with the build.

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1) Were there any speed-related changes between 1.3.94 and 1.3.105? I ask because the external Synocommunity package for Synology DiskStation devices has not yet been updated to 105, so I am running 94 on my NAS.

2) Is there a changelog made public somewhere?

3) Using BT Sync with two devices, a server in North America and my NAS in Melbourne, Australia; speeds are slow*. I average about 300KB/s with BT Sync, but about 1.8–2.1MB/s using multiple connections over FTP (specifically, using lftp). Is this a limitation of the Bittorrent P2P 'swarm' technology, and I am seeing slow speeds because there are only two peers in the 'swarm'?

4) Following on from 3), if I'm speed limited by the number of peers, could we artificially populate one or both links with fake peers? Similar to traditional multiple connections using lftp or aria2, but with the BT P2P tech.

5) Is / will / should it be possible to saturate my bandwidth and maximise my connection speed using BT Sync given the distance between and low number of peers?

* Yes, speeds are this slow/just as slow on a single HTTP link because of the tyranny of distance. I get ~350KB/s over HTTP (single connection).

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Hmm. Well I am moving house next week and disconnecting/reconnecting internet etc. Same ISP though. Once I've moved and reconnected, I will run some tests, do some research and try to find where the bottleneck could be on my side or at the level of my ISP. I will report back here with my findings in about a month.

It's good to hear that even just two peers should saturate a connection, though, because I'd much rather use BT Sync than other solutions such as FTP. I'm very impressed by BT Sync so far, so I hope I can solve these problems.

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The saw shaped bottlenecks are easy to reproduce. Basically if I set the upload speed to let say 100kb and have download set to unlimited. Soon as current upload speeds reach 100kbs boom download and upload speeds all drop to 0. If both upload and download are unlimited, no issue. This only seems to be a issue with the upload being maxed out(for me at least). As a work around I've basically had to block peers I do not want to send to, as that it affects download speeds. Personally I feel upload limits should not affect download speeds. Also note I'm running 1.4.103

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Soon as current upload speeds reach 100kbs boom download and upload speeds all drop to 0. If both upload and download are unlimited, no issue. This only seems to be a issue with the upload being maxed out(for me at least).

I'm not uploading anything and no process is either:

@RomanZ it tops out at 3.63MB/s and 3.6MB/s is the average.

Here is a video of a 200MB test file:

(You will need to select 1080p/720p and go fullscreen as it's cropped)

The connection between my dedicated server and home is solid from start to finish over FTP. Using BTSync it's a never-ending wall of sawtooths.